Dysregulated expression of the c-MYC proto-oncogene is one of the most frequent abnormalities in human malignancies. Through the control of an expansive target gene network, this transcription factor drives proliferation and, in some settings, induces cell death. Despite great advances in the identification of c-Myc-regulated genes, the mechanisms through which this oncogene promotes tumorigenesis are not yet fully understood. MicroRNAs are ~18-24 nucleotide RNA molecules that have emerged as major regulators of eukaryotic gene expression. We recently identified a group of microRNAs, known as the mir-17 cluster, that are directly upregulated by c-Myc. Independently, these microRNAs were shown to cooperate with c-Myc in promoting tumorigenesis. We have now also identified a cohort of microRNAs that are directly repressed by c-Myc. Many of these are known to be deleted or mutated in cancer, suggesting that they possess tumor suppressor activity. We now propose studies designed to test the hypothesis that these c-Myc-regulated microRNAs are critical components of the c-Myc target gene network that regulate cellular proliferation, apoptosis, and neoplastic transformation. In Aim 1 of this project, we will investigate the phenotypic consequences of expression or inhibition of the mir-17 cluster in multiple cell types. These experiments are necessary to set the stage for detailed molecular analyses of the mechanisms through which these microRNAs influence oncogenic phenotypes. We will begin to investigate these mechanisms in the latter part of this aim, where the hypothesis that the mir-17 cluster influences cell-cycle control through regulation of p21WAF1 is tested. In Aim 2, we will continue to dissect the mechanisms through which the mir-17 cluster influences cellular phenotypes. Here, we will test the hypothesis that the mir-17 cluster participates in a physiologic negative-feedback circuit that maintains tightly-controlled expression of c-Myc and other Myc family members in non-transformed cells. Finally, in Aim 3, we will investigate the role of microRNA repression in c-Myc-mediated tumorigenesis. MicroRNAs that are directly repressed by c-Myc will be characterized and the consequences of enforced expression of these microRNAs in in vitro and in vivo models of lymphoma will determined. We envision that these experiments will reveal novel regulatory circuitry that functions abnormally in cancer and may ultimately be amenable to therapeutic intervention. [unreadable] [unreadable] [unreadable]